Protobuffs compiler take more opts other than just erl_opts

Basically, this issue was brought up by https://github.com/rebar/rebar/pull/150 but it failed to get merged back. I did some improvements on that pull request and Im trying to bringing it up again since I believe it is pretty important for some ppl like me.
Sure, the commit message was changed, please let me know if it looks good to you